Slow wave circuit and method of fabricating same

ABSTRACT

A ceramic supported slow wave circuit for a microwave tube is disclosed. A slow wave circuit includes a serpentine-shaped ribbon of a refractory metal, preferably rhodium, bonded to the surface of a similarly serpentine-shaped ceramic support structure. The serpentine-shaped ceramic support structure is in turn bonded to a metallic support member having a coefficient of thermal expansion matching that of the ceramic support. A composite ring and bar circuit is formed by disposing two of such serpentine-shaped semicylindrical curved slow wave circuits in mutually opposed relation and in transverse registration with each other. The slow wave circuit is conveniently deposited on the dielectric support structure either before or after slotting the ceramic support from opposite sides to form the serpentineshaped dielectric support.

United States Patent [72] Inventors Louis R. Falce Redwood City; Allan W. Scott, Los Altos, both of Calif. [21] Appl. No. 8,793 [22] Filed Feb. 5, 1970 [45] Patented Oct. 5, 1971 [73] Assignee Varian Associates Palo Alto, Calif.

[54] SLOW WAVE CIRCUIT AND METHOD OF FABRICATING SAME 13 Claims, 8 Drawing Figs.

[52] US. Cl 3l5/3.5, 29/600, 315/39.3,29/25.17 [51] Int. Cl l-lOlj 25/34 [50] Field of Search 29/600, 601; 315/35, 3.6, 39.3

[56] References Cited UNITED STATES PATENTS 3,373,382 3/1968 Diamand 333/31 A 3,382,399 5/1968 Garoff 3l5/3.S

Primary Examiner-Herman Karl Saalbach Assistant Examiner-Saxfield Chatmon, Jr. Anurne \-sStanley Z. Cole and Gerald L. Moore ABSTRACT: A ceramic supported slow wave circuit for a microwave tube is disclosed. A slow wave circuit includes a serpentine-shaped ribbon of a refractory metal, preferably rhodium, bonded to the surface of a similarly serpentineshaped ceramic support structure. The serpentine-shaped ceramic support structure is in turn bonded to a metallic support member having a coefficient of thermal expansion matching that of the ceramic support. A composite ring and bar circuit is formed by disposing two of such serpentineshaped semicylindrical curved slow wave circuits in mutually opposed relation and in transverse registration with each other. The slow wave circuit is conveniently deposited on the dielectric support structure either before or after slotting the ceramic support from opposite sides to form the serpentineshaped dielectric support.

PAIENIEDIJCI slsn SHEET 1 BF 2 FIG. 5

M E l N R T wDH I M I DI I TL PATTIOWDIRCL DITHTIIL DITIIHI HAINT ULIIITOROR EUE HETII l UED EG II W SIFEPU R CI L E U m mmssPT M W MFRW %M G U0 T I I On n N R ER EL HU MSCLVO R M M H I NW M N WA EA Cl N m v EU F. J S I WE I E HUT H NMS E F ISVDL C M RF IT NFTLT H 0 OANE L D PA U U H MDIR N DHDIC NRRTT M m L PRM E OrrL U VAH OTIE Im SMOAQ AmD SA E DDSSAMWCIT C INVENTORS LOUIS R. FALCE ALLAN w. soon BY %uu&l 3mm ATTORNEY SLOW WAVE CIRCUIT AND METHOD OF FABRICATING SAME DESCRIPTION OF THE PRIOR ART Heretofore, it has been proposed to form a composite ring and bar slow wave circuit by depositing a conductive metal layer into a semicylindrical trough in a ceramic support member. The metallic layer and the support member were then slotted in such a manner as to leave semicircular ring segments and interconnecting bar portions all supported from the ceramic supportJ-Iowever, to from a ring and bar composite slow wave circuit two of such semicylindrical circuits had to be brazed together at mating tab portions on'the semicircular rings. Such a circuit and method of fabricating same is disclosed and claimed in copending U.S. .Ser. No. application 609,522 filed Jan. 16, 1967 now U.S. Pat. 3,505,730 and assigned to the same assignee as the present invention. While this aforedescribed circuit substantially extended the thermal capacity and, thus, the operating power level of tubes using the circuit, as compared to copper circuits of equivalent size which were not completely supported from the ceramic support, a limiting operating temperature was encountered, namely, the temperature at which the relatively low melting point brazing alloy began to evaporate within the tube at relatively low pressures encountered therein, such as torr. More specifically, operating temperatures for the slow wave circuit were limitedto approximately 600 0. due to evaporation of the brazing alloy. Thus, it is desirable to provide an improved ceramic supported ring and bar circuit which may be made of a refractory metal and which does not require the use of brazing alloys.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved slow wave circuit .for microwave tubes and methods of fabricating same.

One feature of the present invention is the provision of a slow wave circuit for use in microwave tubes which includes a serpentine-shaped refractory metallic ribbon bonded to .a serpentine-shaped surface of a ceramic support structure for supporting the circuit without the use of brazing alloys.

Another feature of the present invention is the same as the preceding feature wherein the serpentine-shaped ceramic support structure is in turn bonded to a metallic body made of a material having a thermal coefficient of expansion matching that of the ceramic.

Another feature of the present invention is the same as any one or more of the preceding features wherein a pair of such serpentine-shaped circuits supported upon serpentine-shaped dielectric support structures are disposed one above the other in transverse registration to define a ring and bar circuit.

Another feature of the present invention is a method for fabricating the slow wave circuit of the first feature wherein the refractory metallic material to form the circuit is deposited upon a surface of the ceramic support structure and the ceramic support structure is slotted from opposite sides with a pair of interdigitated array of slots forming the. serpentineshaped circuit and serpentine-shaped ceramic support structure.

Another feature of the present invention is a method for fabricating the circuit of the first feature wherein a ceramic support structure is slotted from opposite sides with a pair of interdigitated arrays of slots to form a serpentine-shaped ceramic support structure and then a refractory metal layer is deposited on a serpentine-shaped face of the support structure to provide a serpentine-shaped slow wave circuit bonded to the serpentine-shaped dielectric support structure.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawingswherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a dielectric support structure having a metallic layer deposited thereon,

FIG. 2 is a view of the structure of FIG. 1 taken along line 2-2 in the direction of the arrows and being further modified to include a pair of interdigitated arrays of slots to define a serpentine-shaped structure.

FIG. 3 is a sectional view of the structure of FIG. 2 taken along line 3-3 in the direction of the arrows and modified to include a metallic support member bonded to the serpentineshaped structure,

FIG. 4 is a schematic perspective view of a microwave tube incorporating a slow wave circuit of thepresent invention,

"FIG. 5 is a flow diagram, in block diagram form, depicting the method of fabricating the slow wave circuit of the present invention, I

FIG. 6 is a transverse view of an alternative serpentineshaped slow wave circuit of the present invention,

FIG. 7 is a sectional view of the structure of FIG. 6 taken along line 7-7 in the direction of the arrows, and 1 FIG. 8 is an enlarged sectional view of the structure of FIG. 3 taken along'line 8--8;in the direction of the arrows and modified to show an alternative slot configuration.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1-5, the slow wave circuit of the present invention and the method of fabricating same will be described. In the first step of the method, (See FIGS. 1 and 5) an elongated dielectric support member 1 which is preferably made of a refractory thermally conductive material, such as alumina or beryllia ceramic, having one face 2 thereof shaped to conform to the shape of the slow wave circuit portion to be fabricated. In the caseof a slow wave circuit for use in a linear beam tube with a pencil-shaped beam and with the circuit surrounding the bearn, face 2 is preferably sernicylindrical and concave with the axis .of revolution of the semicylindrical section being parallel to the longitudinal axis of the circuit to be formed thereon. A layer of a refractory metallic material, such as molybdenum, tungsten or rhodium, is bonded to face 2 without the provision of an intermediary brazing alloy, such as copper-gold eutectics. More particularly, the metallic layer 3 is conveniently bonded to the dielectric support 1 by sputter depositing a relatively thin layer of molybdenum or tungsten to a thickness of- 400 l0,000. The molybdenum or tungsten metal forms a tightly adhering layer 3. Rhodium is then electroplated over the base layer to a thickness, as of 00002-0003 inches." Rhodium is favored as the outer portion of the metallic layer 3 because it has the lowest resistance of refractory metals thatcan be conveniently electroplated and it has a relatively high melting temperature, as of I960 C. and is capable of operating to l 350 C. in a vacuum of IO torr before beginning to evaporate. This is to be contrasted with copper which evaporates at l0 torr at the temperature of 680 C. Alternatively, rhodium may be evaporated into the bare dielectric, or onto a base layer of M0 or W to the desired thickness.

In the second step of the method (see FIG. 2), the dielectric support member I .is transversely slotted with two arrays of slots 4 extending into. the ceramic body I from opposite sides in an interdigitated fashion to define a serpentine-shaped slow wave circuit anda serpentineshaped dielectric support structure. The resultantserpentine-shaped ribbon slow wave circuit portion is disposed in intimate Contact with and is supported over its entirety by the ceramic support I. The slotting is conveniently accomplished by use of ganged diamond loaded grinding wheels although other techniques may also be employed such as ganged reciprocating blades and the like.

In the third step of the method (see FIG. 3), the face of the serpentine-shaped support structure I is brazed to an elongated metallic support member 5 made of a material having a coefficient of thermal. expansion substantially equal to that of the serpentinesshap'ed dielectric support I. The typical material for the support member 5, for brazing to alumina or beryllia ceramic support I, is a composite material marketed under the trademark Elkonite and formed of a porous tungsten matrix having the pores thereof infiltrated with copper.-The

matrix is comprised of approximately 65 percent tungsten by volume and 35 percent copper by volume. A conventional molybdenum-manganese metalizing compound may be applied to the serpentine-shaped face of the support 1 which is opposite to that containing the circuit 3 and which is to be brazed to the support 5. Conventional copper-gold brazing alloy is then employed for brazing the metalized serpentineshaped ceramic l to the support member 5. The reason that a relatively low melting point brazing alloy mat be employed at this latter braze is because the support member 5 either forms a portion of the envelope of the microwave tube or is in intimate contact with the envelope, such that thermal energy is being rapidly removed and as a consequence this brazed joint operates at a temperatures well below the melting point of the brazing alloy when the circuit portion 3 is operating near its maximum operating temperature such as l200 C.

In the fourth step of the method (see FIG. 4), a pair of the serpentine-shaped dielectric support 1 and metallic support member 5, are brazed as indicated at 8 into a barrel 7 of a vacuum envelope of a microwave tube. The two circuit halves are disposed in mutually opposed relation with the serpentineshaped circuits 3 facing each other in transverse registration to form a ring and bar composite slow wave circuit. The two halves of the circuit 3 need not make physical or electrical contact with each other and, in a preferred embodiment, the two halves of the circuit are spaced apart by an amount approximately on the order of IO percent of the diameter of the composite circuit. At the joint 8 between the conductive support member 5 and the barrel 7, the brazing alloy to be employed should have a brazing temperature below the temperature used at joint 6.

The microwave tube is then assembled and the circuit 3 is axially aligned with an electron beam 9 projected from an electron gun ll centrally through the composite circuit along the central axis thereof to a collector electrode 12 at the terminal end of the tube for dissipating the energy of the beam. If the tube is an amplifier, suitable input RF connections are made to the composite circuit for feeding RF energy onto the circuit and suitable conventional connections are made near the downstream end of circuit for extracting amplified RF energy.

The advantage of the slow wave circuit of the present invention is that a refractory metal circuit which is supported at all points by a thermally conductive ceramic support structure is produced which does not require the use of brazing alloys for bonding the circuit to the dielectric support structure. Therefore, the operating temperature of the circuit can be raised well above the temperature of similar circuits utilizing brazing alloys. A ring and bar circuit of the type depicted in FIG. 4 readily permits the tube to operate with an output power of 5 kilowatts CW at C-band.

Although the barrel 7 of the tube has been shown of rectangular cross section, this is not a requirement. The barrel 7 may be a cylindrical tube of copper or may be made by a brazed stack of annular iron pole pieces of annular cupronickel spacers to permit periodic permanent magnet focusing of the beam 9.

In the method of FIG. 5, the metallic layer 3 need not be deposited upon the dielectric support structure 1 before the transverse slotting to produce the serpentine-shaped for the circuit and dielectric support. More particularly, the ceramic member 1 may be transversely slotted to produce the serpentine-shape and then the metallic layer may be deposited upon the appropriate face of the serpentine support structure, as shown in FIG. 2.

The serpentine-shaped circuit 3 is also known in the art as a meander line and is a useful well-known slow wave circuit in its own right. It may be employed, for example, as the slow wave circuit in a crossed field tube either of the linear or circular type. In case the circuit is to be utilized in a linear crossed field tube, the serpentine circuit 3 would be deposited or formed upon a flat face of the dielectric support structure as opposed to a curved face as described above with regard to FIGS. 1-4.

In case that the circuit is to be employed in a tube of circular format (see FIGS. 6 and 7), the conductive metallic layer 3 is conveniently deposited upon the inside cylindrical surface of a toroidal-shaped ceramic support 1. The ceramic support 1 is then slotted by two arrays of axially directed slots 4 which are interdigitated in the axial direction to define a meander line circuit on the inside cylindrical surface of the toroidal ceramic 1. Excess metallic material is preferably removed from the axial end regions 14 to permit brazing of the toroidalshaped serpentine dielectric support 1 to metallic disks 15 at 16.

Disks 15 preferably have a coefficient of thermal expansion matching the ceramic 1. The disks are in turn brazed to a cylindrical body portion 17 forming, a portion of the vacuum envelope of the tube. A sector portion of the toroid l, at 18, is preferably not slotted in order to define a circuit sever portion 19 of the composite slow wave circuit. RF input and output connections 21 and 22, respectively, are made to the circuit 3 for feeding RE energy onto the circuit and extracting amplified RF energy from the downstream end thereof. A thermionic cathode emitter 23 is centrally located of the circuit 3 to define a magnetron interaction region 24 in the region between the cathode 23 and the anode circuit 3. An axially directed magnetic field is provided, by means of a magnet, now shown, in order to obtain crossed field interaction.

Although the dielectric support body 1 thus far has been shown and described as transversely slotted to form a serpentine-shaped dielectric support structure, this is not a requirement. It is only necessary that the circuit support face 2 of the dielectric body 1 be serpentine-shaped. Accordingly, the interdigitated arrays of slots 4, from opposite sides of the dielectric body 1, need not pass through the base of the body 1, where it joins the base support 5, as shown in F IG. 8. In such a case the slots 4 may be semicircular with a radius of curvature larger than that of the semicircular circuit rings portions 3 defined between adjacent slots 4.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a method for fabricating a dielectrically supported slow wave circuit for a microwave tube the steps of, bonding a layer of metal to a dielectric support body, slotting into the dielectric body and metal layer from a first side of the dielectric body with an array of slots which extend only partially through the dielectric body, and slotting into the dielectric body and metal layer from a second side of the dielectric body opposed to the first side with an array of slotswhieh extend only partially through the dielectric body and which are interdigitated with the slots of the first array to define a continuous, generally serpentine-shaped slow wave circuit supported from a generally serpentine-shaped face of the dielectric support structure.

2. The method of claim 1 including the step of bonding a side of the dielectric support structure which is spaced from the slow wave circuit to a metal body having a coefficient of thermal expansion substantially the same as that of the material of the dielectric support body.

3. The method of claim 1 including the step of forming a generally curved concave face in the dielectric body, and wherein the step of bonding the metal layer to the dielectric body includes the step of bonding the layer to the curved concave face of the dielectric body.

4. The method of claim 1 including the step of making a second one of the serpentine-shaped slow wave circuits bonded to a continuous serpentine-shaped face of a second dielectric support body, and positioning the two continuous serpentine-shaped slow wave circuits in mutually opposed noncontacting spaced relation facing each other, such spacing being small compared to the transverse registration with each other such that the unslotted and slotted portions of the slow wave circuits are in transverse registration with each other, whereby a composite slow wave circuit is obtained.

5. The method of claim 4 including the step of forming a generally semicylindrical concave face in both dielectric bodies with the axis of revolution of the concave face extending longitudinally of the elongated dielectric bodies, and wherein the step of bonding the metal layers to the dielectric bodies includes the step of bonding the layers to the semicylindrical concave faces of the dielectric bodies, whereby a composite ring and bar circuit is obtained.

6. In a microwave tube apparatus, slow wave circuit means for electromagnetic interaction with a stream of electrons, the improvement wherein, said slow wave circuit means includes an electrically conductive metallic ribbon formed in continuous, generally serpentine-shape with the convolutions of the serpentine being in the surface of a wide face of said metallic ribbon, a dielectric support structure having a continuous serpentine-shaped face conforming to the serpentine-shape of and disposed in transverse registration with said serpentineshaped metallic ribbon and being bonded to a wide face of said metallic ribbon for supporting said ribbon.

7. The apparatus of claim 6 including a metallic support structure having a coefficient of thermal expansion substantially the same as that of said dielectric support structure, and wherein a side of said dielectric support structure which is opposite said continuous serpentine-shaped metallic ribbon is bonded to said metallic support structure.

8. The apparatus of claim 6 wherein said surface of said continuous dielectric support which contains said serpentineshaped metallic ribbon is a curved concave surface.

9. The apparatus of claim 6 wherein said metallic ribbon is made of a refractory metal material having a melting point above 1900 C.

10. The apparatus of claim 9 wherein the refractory metal material is rhodium.

11. The apparatus of claim 8 wherein said slow wave circuit includes a second continuous serpentine-shaped metallic ribbon bonded to a continuous serpentine-shaped face of a dielectric support body, and wherein said first and second metallic ribbons are disposed in mutually opposed nonelectrically contacting relation facing each other in substantially transverse conforming registration such that similar parts of each ribbon are in transverse alignment with each other to form two halves of a composite slow wave circuit.

12. In a method for fabricating a dielectrically supported slow wave circuit for a microwave tube the steps of, slotting into a dielectric support body from a first side thereof with an array of slots which extend only partially through the dielectric body, slotting into the dielectric body from a second side of the body opposed to the first side with an array of slots which extend only partially through the dielectric body and which are interdigitated with the slots of the first array to define a continuous generally serpentine-shaped face for the dielectric support structure, and deposition a layer of metal onto the serpentine-shaped face of the dielectric support structure to form a dielectrically supported serpentine-shaped slow wave circuit.

13. The method of claim 12 including the step of bonding a side of the slotted dielectric support structure which is disposed opposite slow wave circuit to a metal body having a coefficient of thermal expansion substantially the same as that of the dielectric material of the support body. 

1. In a method for fabricating a dielectrically supported slow wave circuit for a microwave tube the steps of, bonding a layer of metal to a dielectric support body, slotting into the dielectric body and metal layer from a first side of the dielectric body with an array of slots which extend only partially through the dielectric body, and slotting into the dielectric body and metal layer from a second side of the dielectric body opposed to the first side with an array of slots which extend only partially through the dielectric body and which are interdigitated with the slots of the first array to define a continuous, generally serpentine-shaped slow wave circuit supported from a generally serpentine-shaped face of the dielectric support structure.
 2. The method of claim 1 including the step of bonding a side of the dielectric support structure which is spaced from the slow wave circuit to a metal body having a coefficient of thermal expansion substantially the same as that of the material of the dielectric support body.
 3. The method of claim 1 including the step of forming a generally curved concave face in the dielectric body, and wherein the step of bonding the metal layer to the dielectric body includes the step of bonding the layer to the curved concave face of the dielectric body.
 4. The method of claim 1 including the step of making a second one of the serpentine-shaped slow wave circuits bonded to a continuous serpentine-shaped face of a second dielectric support body, and positioning the two continuous serpentine-shaped slow wave circuits in mutually opposed noncontacting spaced relation facing each other, such spacing being small compared to the transverse registration with each other such that the unslotted and slotted portions of the slow wave circuits are in transverse registration with each other, whereby a composite slow wave circuit is obtained.
 5. The method of claim 4 including the step of forming a generally semicylindrical concave face in both dielectric bodies with the axis of revolution of the concave face extending longitudinally of the elongated dielectric bodies, and wherein the step of bonding the metal layers to the dielectric bodies includes the step of bonding the layers to the semicylindrical concave faces of the dielectric bodies, whereby a composite ring and bar circuit is obtained.
 6. In a microwave tube apparatus, slow wave circuit means for electromagnetic interaction with a stream of electrons, the improvement wherein, said slow wave circuit means includes an electrically conductive metallic ribbon formed in continuous, generally serpentine-shape with the convolutions of the serpentine being in the surface of a wide face of said metallic ribbon, a dielectric support structure having a continuous serpentine-shaped face conforming to the serpentine-shape of and disposed in transverse registration with said serpentine-shaped metallic ribbon and being bonded to a wide face of said metallic ribbon for supporting said ribbon.
 7. The apparatus of claim 6 including a metallic support structure having a coefficient of thermal expansion substantially the same as that of said dielectric support structure, and wherein a side of said dielectric support structure which is opposite said continuous serpentine-shaped metallic ribbon is bonded to said metallic support strucTure.
 8. The apparatus of claim 6 wherein said surface of said continuous dielectric support which contains said serpentine-shaped metallic ribbon is a curved concave surface.
 9. The apparatus of claim 6 wherein said metallic ribbon is made of a refractory metal material having a melting point above 1900* C.
 10. The apparatus of claim 9 wherein the refractory metal material is rhodium.
 11. The apparatus of claim 8 wherein said slow wave circuit includes a second continuous serpentine-shaped metallic ribbon bonded to a continuous serpentine-shaped face of a dielectric support body, and wherein said first and second metallic ribbons are disposed in mutually opposed nonelectrically contacting relation facing each other in substantially transverse conforming registration such that similar parts of each ribbon are in transverse alignment with each other to form two halves of a composite slow wave circuit.
 12. In a method for fabricating a dielectrically supported slow wave circuit for a microwave tube the steps of, slotting into a dielectric support body from a first side thereof with an array of slots which extend only partially through the dielectric body, slotting into the dielectric body from a second side of the body opposed to the first side with an array of slots which extend only partially through the dielectric body and which are interdigitated with the slots of the first array to define a continuous generally serpentine-shaped face for the dielectric support structure, and deposition a layer of metal onto the serpentine-shaped face of the dielectric support structure to form a dielectrically supported serpentine-shaped slow wave circuit.
 13. The method of claim 12 including the step of bonding a side of the slotted dielectric support structure which is disposed opposite slow wave circuit to a metal body having a coefficient of thermal expansion substantially the same as that of the dielectric material of the support body. 